Fix warnings summary header appearing twice

Ref: https://github.com/pytest-dev/pytest/pull/4450#discussion_r236017645
Ref: https://github.com/pytest-dev/pytest/pull/4399

doc/en/warnings.rst

@@ -33,8 +33,6 @@ Running pytest now produces this output::
       $REGENDOC_TMPDIR/test_show_warnings.py:4: UserWarning: api v1, should use functions from v2
         warnings.warn(UserWarning("api v1, should use functions from v2"))
 
-    -- Docs: https://docs.pytest.org/en/latest/warnings.html
-    ========================= warnings summary (final) =========================
     -- Docs: https://docs.pytest.org/en/latest/warnings.html
     =================== 1 passed, 1 warnings in 0.12 seconds ===================
 
@@ -358,8 +356,6 @@ defines an ``__init__`` constructor, as this prevents the class from being insta
       $REGENDOC_TMPDIR/test_pytest_warnings.py:1: PytestWarning: cannot collect test class 'Test' because it has a __init__ constructor
         class Test:
 
-    -- Docs: https://docs.pytest.org/en/latest/warnings.html
-    ========================= warnings summary (final) =========================
     -- Docs: https://docs.pytest.org/en/latest/warnings.html
     1 warnings in 0.12 seconds
 

doc/en/writing_plugins.rst

@@ -422,8 +422,6 @@ additionally it is possible to copy examples for an example folder before runnin
       $REGENDOC_TMPDIR/test_example.py:4: PytestExperimentalApiWarning: testdir.copy_example is an experimental api that may change over time
         testdir.copy_example("test_example.py")
 
-    -- Docs: https://docs.pytest.org/en/latest/warnings.html
-    ========================= warnings summary (final) =========================
     -- Docs: https://docs.pytest.org/en/latest/warnings.html
     =================== 2 passed, 1 warnings in 0.12 seconds ===================